Spark plug electrode and other electron emitting device



DEVICES Aug. 16, 1932 v 0.5. DUFFENDACK ETAL ARK PLUG ELECTRODE ANDEMITTING Filed March 2, i931 Patented Aug. 16, 1932 UNITED STATES PATENTOFFICE ORA S. DUFFENDACK, OF AN N ARBOR, AND HECTOR RAIBEZZANA, 0FFLINT, MICHIGAN, ASSIGNORS TO A C SPARK PLUG COMPANY, OF FLINT,MICHIGAN, A COMPANY OF MICHIGAN SPARK PLUG ELECTRODE AND OTHER ELECTRONEMITTING DEVICE Application filed March 2,1931, Serial No. 519,652, andin Great Britain October 11, 1928.

This is a continuation in part of our application Serial Number 302,904,filed August 30, 1928.

Spark plugs, such as are commonly used upon internal combustion engines,consist essentially of a pair of electrically conducting elementsseparated by an insulator. The ends of these elements called electrodes,are spaced in such a way as to provide a gap across which the electricspark is caused to jump to ignite the combustible mixture.

One of the principal difficulties encoun tered with spark plugs is thefact that the coating produced by the combustion gases j over theexposed part of the plug may, in

time, interfere with the ignition. The coating consists of variousby-products of combustion, such as carbon, iron oxide, and the like, aswell as special compounds sometimes formed where anti-knock fuels areemployed. The coating increases as the spark plug is used and, as it isa relatively good conductor of electricity, in time provides aconducting path parallel to the path across the gap, of

- sufiiciently high conductance so that the current flows through thiscoating instead of jumping the gap between the electrodes. The result isthat the engine misses,--that is, some cylinder fails to give powerimpulses because no spark is delivered at the gap to ignite thecombustible mixture.

In the present day ignition equipment a spark coil or magneto suppliesthe sparking voltage. The design of this equipment is necessarily suchthat the available voltage falls off abruptly with increase in theconductance along the path constituted by the coating on the insulator.After the initial sharp reduction, the voltage dropsmore gradually withincrease in the conductance along the s'a'id path. As a result of thischaracteristic of the ignition apparatus, it is apparent that a plugrequiring a high sparking voltage will become fouled very much morequickly thana plug requiring a low sparking voltage; In the case of onetypical ignition system, in a test in wh ch plugs were operated underidentical conditions, it was found that while a plug having a sparkingvoltage of 8,000 volts ceased to fire the combustible mixture of theengine when enough coating had collected on the insulator to form aleaka e path of 1.5 micromhos, a plug having a spar ing voltage of 4,000continued to operate untll theleakage path reached a value of 10micromhos. Since it would take several times as long for enough tocollect on a plug to increase the conductance of the leakage path to 10micromhos as it does to reach a value of 1.5 micromhos, the plugoperating at the lower voltage will give satisfactory service for a farlonger perlod of time than the other. The importance of operating theplug at low voltages is clearly apparent.

It has been known, heretofore, that the sparking voltage varies with thelength of the gap and the density of the gases in the combustion chamberatthe instant of ignition, and with the temperature of the electrodes.It was also known that the sparkin voltage varies with the shape of theelectro es but it has heretofore been universally accepted that thesparking voltage was practically independent of the materials employedin the composition of the electrodes. However, upon testing a largenumber of spark plugs of identical construction under the sameconditions of gap width, gas density, and electrode temperature, wenoted considerable variation in the sparking voltage that could not besatisfactorily explained.

We also knew, from past experience, that the voltage required to cause aspark to pass between electrodes of a spark plug gradually increaseswith time and use, and eventually becomes higher than the maximum thatthe ignition coil will generate. This effect is exaggerated under coldweather starting conditions- No satisfactory explanation of either ofthe above phenomena was advanced but it remained the fact thatapparently identical plugs, operated under identical conditions, wouldspark at widely varying voltages and that the same plug, operated underapparently the same conditions, would require higher voltages to producethe spark as time went on.

The materials, usually metals, of which spark plug electrodes havealways been made are of commercial purity. The electrode metal isusually an alloy made in accordance with a definite formula and, uponchemical analysis, shows the presence of the chosen ingredients only.However, it occurred to us in the same plug after a period of use weredue to the presence in the electrodes of 1m- I purities, widely varyingin amount, but always in such minute traces as to escape detection byordinary chemical analysis. Accordingly a thorough laboratoryinvestigation was made to determine what, if any, change took place inthe composition of the electrode wire with changes in the sparkingVoltage. With the aid of thermionic studies under vacuum and of spectrumanalysis it was found that the spark plug electrode alloy consisting ofapproximately 98% nickel and2% manganese,containedtraces of a; number ofother elements, and among them frequently magnesium; that underconditions of use the magnesium left the alloy at a greater rate thanthe nickel and as the supply of magnesium in the alloy became depletedthe sparking voltage rose. To check this conclusion spark plugs weremade having electrodes from which all traces of magnesium had been.removed by long continued heating in vacuum. Tests of these plugs in allcases showed much higher sparking voltages.

Spark plugs were also made having electrodes containing appreciablepercentages of magnesium as contrasted with the microscopic traces oftenoccurring in the commercial alloy. Upon testing, these plugs showedmarkedly lower sparking voltages, but the voltage varied considerablynot only from spark to spark but even more markedly at pointserratically spaced along the length of the electrode. These variationswe found to be due to the fact that magnesium was present in the wire assmall irregularly spaced inclusions and did not form a true solution oralloy with the nickel. In further confirmation of the accuracy of ourconclusions, we found that as the magnesium was -evapo rated, orotherwise removed from the electrodes, the sparking voltage rises. Thisexplains the rising of the sparking voltage with continued use of theplug. Furthermore, plugs which failed to spark in actual use in anengine were found to have electrodes (of the usual alloy). deficient inmagnesium or entirely lacking that element. these undesirable variationsin voltage with alloys containing magnesium we were led to search formaterials that would better serve our purpose.

After exhaustive research and thorough study of the problem we havefound that there is a definite class of elements that, from theoreticalconsiderations, should, when employed in spark plug electrodes, markedlyreduce the sparking voltages. These elements are Because ofcharacterized by a low Work function. By work function is meant theamount of work necessary to remove an electron from the element. Workfunction may be measured by measuring the thermionic emission and invarious other ways. The elements of low work function are also, ingeneral, characterized by the fact that they are stronglyelectropositive, and include the alkali and alkaline earth metals. Thegroup of elements having low work function comprises lithium, sodium,potassium, rhubidium, caesium, beryllium, magnesium, calcium, strontium,barium, thorium and radium.

Aside from low work function, however, there are other factors whichdetermine the usefulness of elements in spark plug electrode work. Thusthey should be capable of being easily alloyed with, or uniformlydistributed throughout the other elements, such as nickel and/ormanganese, which it is desirable to include in the alloy from the pointof view of electrical conductivity and resistance to heat and corrosion.It is also important that the added elements shall not be readilyremoved from the base material. In other words, it is important that therate of depletion of the element or elements of low work function fromthe alloy shall be low. This insures that the plug will operate for longperiods of time at the desired low voltage.

Our researches to date indicate that of the above group the only onesthat are capable of practical use in spark plug electrodes are thealkaline earth metals, barium, calcium and strontium. The alkali metalshave proven to be impracticable because, in the first place, owing totheir rapid oxidation, it was practically impossible to include enoughof the alkali material in the mixture to be of any value and, in thesecond place, the small amount that was included evaporates from thewire at so great a rate under conditions of use as to have but atemporary effect on the sparking voltage. Thus, a nickel alloycontaining traces of sodium was tested by heating a wire made of thematerial. At the end of an hours heating, no traces of sodium could bedetected even spectroscopically. Further tests of such alloys in sparkplugs showed such an increase in sparking voltage after but a fewthousand sparks that it was evident that practically all the sodium haddisappeared from the wire.

The alkaline earth metals, barium, calcium and strontium, when includedas an integral part of the spark plug electrode, markedly reduce thesparking voltage, and the electrodes retain this property over a longperiod of time. We have previously pointed out the lack of uniformity insparking voltage that characterizes the nickel magnesium alloy. This isobjectionable because ignition is there by rendered irregular, and astimegoes on, the sparking voltage rises and the engine becomes-harder tostart. Nickel wire having strontium intimately distributed throughoutit, has much the same characteristics as the nickel magnesium wire. Thestrontium and nickel do not form a solid solution or alloy, thestrontium occurring as inclusions throughout the wire. While thesparking voltage is reduced by the presence of strontium, there isconsiderable lack of uniformity in the sparking voltage.

Barium and calcium, on the other hand, alloy Well with nickel, formingtrue solid solutions. As a consequence the sparking voltage remainsuniform. From experiments we have made, however, We are led to believethat of the two the barium alloy is much to be preferred for its rate ofevaporation from the alloy is much less, so that the barium not onlygives a large reductionein sparking voltage but maintains that propertyalmost indefinitely.

' When any of the alkaline earth elements, calcium, strontium or barium,is included in a spark plug electrode composition, in an amount even aslow as one one-hundredth of one per cent, a large and'more or lesspermanent reduction in sparking voltage will result, amounting to asinuch as 25% in the case of barium; and, at the same time, the variationin voltage between successive sparks is greatly reduced. In practice weprefer to use larger amounts of the low Work function material in orderto secure a greater reduction in sparking voltage, the upper limit ofthe amount used being usually determined by considerations ofconvenience in manufacture and cost.

We believe that the results obtained by the use of these materials inspark plugs may be explained by the electron theory of the conduction ofelectricity through gases. This theory sets forth that in order toproduce a spark, there must be builtup between the electrodes acontinuous flow of electrons. In order to initiate this flow, there isrequired in the ap the presence of a number of free ions. uch free ionsare always found in gases in containers and in the free air. These areaccelerated by the electric field and in traveling toward the electrodescollide with gas molecules at such a rate as to form other free ions bythe impact of their collision. Furthermore, the positive ions collidingwith the cathode, or negative electrode, cause the removal therefrom ofelectrons which pass through the gas to the anode. These electrons inpassing through the gas make collisions with gas molecules and form freeions and electrons by the violence of their impacts. on account of theirrelatively high mobility with respect to free ions, the electronsconstitute the principal carriers of electric charges through the gasbetween .the electrodes. The amount of the current depends principallyupon the number of electrons that can be made to pass per second throughthe gas and the rate of building up of the current depends lar ely uponthe number of electrons that can e removed from the cathode per second.Besides the action of the collisions of positive ions upon the'cathode,other processes are operative to remove electrons from the negativeelectrode, viz: the photoelectric action of light generated in the gapbetween the electrodes by the passage of electric charges or of lightfrom sources external to the spark gap, the thermionic emission ofelectrons from the cathode as a consequence of its being heated bysurrounding bodies or by the heat generated by the positive ionsimpinging upon it, and the action of the electric field in extractingelectrons from the cathode. In each of these processes .energy must beexpended in extracting the electrons in proportion to the work functionof the material of the electrode, and the number of'electrons that canbe removed per second will be increased by the inclusion in the materialof the electrode of substances of low work function, because of therelative ease with which the electrons are extracted from the mass. Ifthe rate of removal of electrons is decreased the rapidly risingignition coil voltage will build to a higher value before the necessaryelectrons are available for the conduction of sufii-. cient current tolimit the rise of voltage. The inclusion in the material of theelectrodes of substances of low work function will reduce the sparkingvoltage because of the increase in the rate of extraction of electronsfrom the cathode and the consequent increase in the rate of rise ofcurrent under the combined action of such of the processes mentionedabove as are operative at any given instant. The differences in rate ofthermionic emission of the materials which occur in the electrode eitherin microscopic traces or in appreciable quantities explains themysterious variations in time lag of the spark that have heretofore beena subject of much conjecture.

While the above is the best explanation of the theory of the operationof these elements according to present day knowledge, it is to beemphasized that the precise character of the phenomenon called theelectric spark is not known, and the above statement is to be consideredin the light of a tentative explanation and not in the nature of alimitation of this invention.

' It is obvious that the above elements will function to produce thedesired result when included in electrodes of widely varyingcomposition; thus, our invention is applicable with electrodes composedof other base metals, such as copper, or of non-metals. However, ingeneral other base materials are not as satisfactory as nickel, eitherbecause of lack of resistance to corrosion or oxidation, or prohibitivecost. It is likewise apparent that our invention is applicable to sparkplugs of whatever design, and, in general, to sparking or ignitiondevices operating under similar conditions. In this field we believe ourinvention to be of pioneer character, for heretofore it had not beenrecognized that the electrode material had any practical eilect onsparking voltage.

In the drawing is illustrated a conventional spark plug provided withelectrodes made in accordance with this invention.

In the figure, we have shown a spark plug including a center electrode4, surrounded by insulator 6 of the usual ceramic composition, whiletheinsulator, in turn, is surrounded by the steel shell 10, carrying theelectrode 8. Either or preferably both of the electrodes 8 and 10 aremade of an alloy or composition including a proportion of one or more ofthe alkaline earth elements above mentioned. As previously pointed out,this proportion may be as low as .01%, but as it is desirable to obtaina greater reduction in sparking voltage a somewhat larger percentageshould be used. The remainder of the composition preferably consistsprincipally of nickel.

A preferred composition of alloy for spark plug electrodes which hasgiven excellent results both from the standpoint of low sparking voltageand uniformity in sparking voltage is given below:

Per cent Manganese 1 8 Barium .03 Magnesium a trace I Balance nickel IThe amount of magnesium present in the alloy is so small, so diflicultto measure by ordinary methods of chemical analysis, and sonon-uniformly distributed that it has been designated as a trace. It isthe residue of about .1% magnesium added to de-oxidize the alloy andincrease its ductility so as to permit easy drawing.

A-barium content of 03% has been chosen for spark plug use because theaddition of sive sparks does not exceed 400 or 500 volts. With thebarium alloy, the plug retains these characteristics throughoutsubstantially its entire life for the barium evaporates from theelectrode at a rate but little greater than that of the nickel.

\Ve have also found that the barium increases the resistance of theelectrodes to corrosion, actual tests show an increase in electrode lifeof 25%.

In preparing the alloy the barium, which is readily oxidized, is sealedwithin a can made of ordinary tin plate, or is wrapped in nickel foiland the package is immersed in the molten nickel-manganese alloy. Thefoil or the can melt and mix with the other ingredients but the amountof foreign material thus introduced is so slight as to have practicallyno measurable efi'ect on the alloy. The advantage of this methodof'preparing the alloy is that the barium is kept out of contact withthe air and becomes alloyed with the nickel and manganese before it hasopportunity to oxidize. Owing to the volatility of the barium, it isdesirable that the temperature of the melt should be but little abovethe melting point of nickel at the time it is introduced and afterintroduction the melt should be allowed to stand before casting onlylong enough to insure complete alloying. However, to secure the maximumbarium content such as is needed in vacuum tubes and the like, and toinsure the least waste of barium, it is best to employ the method ofprocedure described and claimed in the copending application of D. W.Randolph Serial No. 520.704 filed March 6, 1931. By the above describedmethods of preparation we have found that the barium is uniformlydistributed throughout, forming a true solid solution or alloy.

The proportions given above are, as previously stated, not critical. Thepercentages of the ingredients have, for example, been varied within thefollowin g range more barium, while reducing the sparking M Percent e,does not reduce a by an additiona Baqganese amount sufficient to warrantthe expense. A anum to 20 barium content as low as .01% will, however,efi'ect an appreciable lowering of the sparking voltage.

A manganese content around 1.8% will be found adequate for spark pluguses but there may be considerable variation in the percentages of boththis ingredient and of nickel.

Spark plugs having electrodes made of the above alloy have, upon test,shown sparking voltages 25% lower than the average voltages obtainablewith electrodes of the usual nickelmanganese alloy, and whereas with thelatter material the sparking voltage varied as between successivesparks, when operated under the same conditions, by 1,000 to as much as4,000 volts, with our improved electrodes the variation in voltage asbetween succes- Magnesium a trace Balance nickel nique now beingexperimented with, larger percentages of barium may be obtained.

Instead of using nickel as the base metal we may use cobalt, an elementof well recognizedsimilar characteristics. Cobalt will not be, assatisfactory, however, for spark plug work,where corrosionis a factor,for it has relatively poor corrosion resisting properties.

. Elsewhere its use may be desirable. In the broader claims we haverouped nickel and cobalt together. under t e term nickel-cobait metal. yI

We have also made and tested alloys of barium with copper', with siliconand with chromium as base metals.' The copper-barium alloy is not asresistant to corrosion as is desirable for spark plug work. The same istrue of the sihcon-barium alloy. The barium-chromium alloy has proven tobe unworkable by ordinary drawing methods be-' cause of its extremehardness and lack of i ductility; More recent tests have indicated havincopper, silicon or chromium as a base Magnesium a trace that the rateofemission obtained from alloys is so ow comparedfwith nickel or cobaltas .to render them undesirable from a practical particularly seems torestandpoint. Silicon tard emission.

. We have also had silccess with calciumnickel alloys. "While calciumdoes not seem to reduce thes'p'arking volta e as much as barium and alsoevaporates om the electrode more rapidly than barium, it is,nevertheless, next to barium in desirable qualities. Calcium maybealloyed with nickel or with nickel and manganese just as in the caseof barium. The'alloy is prepared in the same manner as the barium alloy.We have found the following. composition to be efiective for spark pluge ectrodes: Per cent Calcmm e Manganese f 2.00

Balance'nickel V y There is nothing critical about the proportionswofthe ingredients except that the calcium should hem excess. of 04% of thetotal to insure that it will have sufiicient' efiect '51. As in the caseof barium,

v for exam le, cobalt, cop r, silicon or chro inium.

in reducing thesparking voltage. 'In actual practice the uantityofcalcium used will be 1n excess of t 's amount; 'We have used up to 10%calcium. In general the reduction in sparking voltage will increase inproportion to the amount of the low work function element that is addedbut it is undesirable to increase the calcium content unnecessarily forreasons of economv. calcium may be employed with base metals other thannickel,

barium.

Strontium ma be combined with nickel in the. metal evaporates from thewire much more rapidly than the other, the continued use of electrodescontaining it is accompanied by increase in sparking voltage due todepletion of the electgon emltting substances.

Many modifications will occur to those skilled in the art, for example,it may prove desirable to employ two or more electron emitting elementsinstead of one in the alloy.

The barium alloys herein disclosed are claimed in our copendinapplication, Serial- No. 574,050, filed Novem er 9, 1931.

We claim: y 1. A-spark plug having an electrode havingv interspersedthroughout its cross section a material of low work function forreducing 'the sparkin voltage.

2. A spar plu having an electrode having interspersedt roughout. itscross section an alkaline earth materialof low work function forreducing the sparking volta I 3. A spark plughavingan electro ecomprising a base material resistant to heat and corrosion and amaterial of low work function distributed throughout the base materialtor reducing the sparking voltage.

4. A spark plug having an electrode formed of nickel-cobalt metal and analkaline earth metal of'low work function for reducing the s arkingvoltage.

' 5. A spar plug havlng an electrode formed of an intimate mixture ofnickel! cobalt metal, manganese, and an, alkaline earth metal of lowwork function for reducing the sparking voltage. 6. A s ark plug havingan electrode formed 0 an alloy of nickel and barium said barium .havingthe property of low work function thereby reducing the sparking voltage.7 7. --A s arkplug having an electrode :formed 0 an alloy of nickel,manganese and barium said barium havin the pro rty of low work functionthere y I reducmg' the sparking voltage. n

In many whereof we afiix our signaese ba'se.meta' however, have thQ'obiections pomted out in connection with the same manner. However,wehave been unable to produce true alloys and owing to p the unevendistribution of this metal in the wire we have not obtained uniformityin .eparking voltage, and owing to the fact that

